GEOLOGY 640: Geology through Global Arts and Artifacts
LAB 5: METAMORPHIC ROCKS
LAYERED METAMORPHIC ROCKS
Most metamorphic rocks formed as a result of deep burial due to tectonic forces
at plate boundaries. Along convergent plate boundaries, plates collide with
sufficient force to raise mountain belts. These regional forces have an effect at
the mesoscopic and microscopic scale as well; when long or flat metamorphic
minerals grow (amphiboles and micas), they do so such that they are narrowest
in the direction that the rock is being squeezed.
Let us examine the case of shale that is metamorphosed due to regional
compression…
Temp
Rock
Name
Rock
Family
Minerals
Present
Arrangement of
Minerals
Rock Appearance
<100°C
Shale
Sed
Clays
Randomly arranged
~100°C
Slate
Met
Clays
Parallel/Stacked
Slightly shiny, breaks along
flat planes (slaty cleavage)
~300°C
Schist
Met
Micas
Parallel/Stacked
Shiny, reflective, sheets of
mica (schistosity)
~600°C
Gneiss
Met
Micas
Layered/Segregated
Banded with light (qtz-feld)
and
dark
(bt-gt)
layers
(gneissosity)
Dull sheen, breaks unevenly
With progressive metamorphism (increasing T), there are several systematic
changes that occur within the rock: 1) minerals change; 2) minerals grow larger;
and 3) minerals align and/or segregate. Note that similar changes will occur in
most rock types; the textures will be similar but the minerals will be different
because each rock type has its unique range of chemical composition. Thus, a
gneiss could form from high-grade metamorphism of shale, granite, or basalt.
(Which would be darkest?)
Quickly re-examine and review the samples of muscovite, biotite, garnet, quartz,
and feldspar. How do you identify mica (muscovite and biotite)?
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1
Examine the following hand samples (140, 150, 159, 200). They represent
various stages in the metamorphism of shale.
Sample
#
Size of
Crystals
Identifiable
Minerals
Arrangement
of Minerals
Rock Appearance
Rock
Name
Arrange these 4 samples (140, 150, 159, 200) in sequence from lowest grade to
highest grade (low T to high T).
____________
Lowest T
____________
____________
____________
Highest T
Explain why slate was commonly used for making blackboards.
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Based on the physical properties of 140, 150, 159, and 200, which rock would be
most appropriate for building a monument? Which would be least appropriate?
Explain your answer with reference to the physical properties.
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2
MASSIVE METAMORPHIC ROCKS
Two of the most common sedimentary rocks, limestone and quartz arenite, are
each composed predominantly of only one mineral, calcite and quartz
respectively. Without other minerals with which to react an all-calcite sedimentary
rock will remain an all-calcite rock when it is metamorphosed. A metamorphosed
limestone is called marble. Similarly, an all-quartz sedimentary rock will remain
an all-quartz rock when it is metamorphosed. A metamorphosed quartz arenite is
called a quartzite.
Compare limestone (120) with marble (161). What characteristics change during
the metamorphic process? What characteristics remain unchanged? Describe
how you can distinguish between a limestone and marble.
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3
Compare sandstone (124) with quartzite (165). What characteristics change
during the metamorphic process? What characteristics remain unchanged?
Describe how you can distinguish between a sandstone and quartzite?
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Describe the physical properties that allow you to distinguish between a marble
and a quartzite. Why might an artist choose to sculpt out of marble? Why might
an artist choose to sculpt out of quartzite?
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4